This invention is in the field of preventing the discoloration of pigments. Discoloration of pigment utilized in paints or coatings, and particularly the discoloration of white pigment, has been recognized to be a severe problem. Optical discoloration has been found to be the most prevalent cause for the degradation of the pigments. Such optical discoloration normally occurs during exposure to ultraviolet radiation. A vacuum environment normally accelerates the discoloration and thus in applications where the pigment is to be exposed to a vacuum environment, such as in space applications and the like, the discoloration problem is more severe. However, changes in the properties of a paint, such as yellowing and chalking due to ultraviolet radiation are, of course, well known in air.
In addition to ultraviolet radiation, pigments have been known to degrade in the presence of X-rays and gamma rays. Normally, however, the degradation does not occur with visible or infrared light, although materials are found that color when exposed to such visible radiation. The discoloration effect is particularly noticeable with while pigments which are normally comprised of metal oxides, though other pigment materials are susceptible to a discoloration, as is well known.
In attempting to overcome the aforegoing problem of discoloration, the prior art technique has simply involved the testing of various pigment materials under ultraviolet radiation conditions to select the one or ones that are most stable against discoloration for use in environments where ultraviolet radiation is a problem. However, no white pigments particularly have been found which are not susceptible to discoloration upon exposure to ultraviolet radiation. Zinc oxide, for example, is a relatively stable pigment, yet it is subject to degradation. There has previously been developed a system of firing ZnO together with SiO.sub.2 to provide improvement in the degradation of the ZnO. The reason for improved results is not clear. Some materials like ZrO.sub.2 have been helped substantially by removing bulk impurities which accelerate the discoloration or degradation. The rate of discoloration of paints in air is very sensitive to the complete system used, some binders being very sensitive to irradiation. Thus, improvement can be obtained using inert binders.
Thus, an object of this invention is to provide a means for preventing the discoloration of pigments.
Another object of this invention is to provide a means for depositing small concentrations of impurities onto the surface of pigment material to prevent degradation of pigment particles or their binder upon exposure to ultraviolet radiation.